The present invention generally relates to direct wire monitoring systems of the type which monitor a direct current supplied from a central station to remote premises via a communications line. More specifically, it relates to circuitry for automatically indicating at the remote premises that the central station is acknowledging reception of a change in the current level from a first normal level to a second normal level.
A typical direct wire monitoring system utilizes a lower value nighttime current than the daytime current. The change in current is produced by manual switching of resistances at the remote premises, which is generally performed by the last person to leave the protected premises in the evening. After switching the resistances and before leaving the premises, he waits for a ringback signal from the central station via the communications line acknowledging reception of the normal nighttime current.
In the prior art, an operator at the central station, when made aware of the change to normal nighttime current, manually switched an alternating current source onto the communications line, ringing a buzzer at the remote premises to ackowledge the change. Since the central station operator monitored hundreds of such systems, and a large number of subscribers tended to switch to nighttime current in a short time interval in late afternoon, there might be a long delay before the central station operator could ring back.
A U.S. patent to Damon, No. 3,854,127, discloses an alarm system which automatically sounds a ringback buzzer at remote premises after a change from normal daytime to normal nighttime current is detected at the central station. As disclosed, two power sources are required: a dc source for supplying monitoring current and a 20 Hz ac source to activate the buzzer. The ac source is switched onto the communications line when it is desired to ring the buzzer.